This invention relates generally to satellite-based communications systems and, in particular, to satellite communications systems that employ a constellation of nongeosynchronous orbit (NGSO) satellites, a plurality of user terminals, and a plurality of gateways for interconnecting voice or data calls between individual ones of the user terminals and terrestrial communication networks via one or more satellites of the NGSO constellation.
U.S. Pat. No. 5,619,525, xe2x80x9cClosed Loop Power Control for Low Earth Orbit Satellite Communication Systemxe2x80x9d, by R. A. Wiedeman and M. J. Sites, describes a satellite communication system that employs terrestrial gateways and a constellation of NGSO satellites, such as Low Earth Orbit (LEO) satellites, for linking user terminals to, for example, the Public Switched Telephone Network (PSTN). Each gateway has a gateway coverage or service area and provides services to those user terminals that reside permanently within the service area, or that reside temporarily (roamers) within the service area. The service area is determined by many factors including, satellite footprint, availability, geometry, national borders, etc. A call that arrives at the gateway from the PSTN for a given user terminal is connected to the user terminal via one or more satellites. In like manner a call originated by the user terminal is connected to the PSTN through the gateway and one or more satellites. This system employs a Code Division Multiple Access (CDMA) modulation scheme, or other multiple access schemes, and enables the gateway to set up multiple links to and from the user terminal through two or more satellites using two or more gateway antennas which may be directional or omni-directional. This provides satellite diversity, as the user terminal (and gateway) are enabled to coherently combine two or more received signals, thus mitigating the effects of signal blockage and fading.
Referring to FIG. 1, the above-referenced U.S. patent applications Ser. No. 08/903,166, filed Jul. 3, 1997, and Ser. No. 08/239,750, filed May 9, 1994, generally teach a satellite communications system wherein a gateway (GW) 1 bidirectionally communicates with a user terminal (UT) 3 via one or more NGSO satellites (e.g., SAT 1 and SAT 2). SAT 1 and SAT 2 may be bent pipe repeaters, or may use on-board processing and may contain regenerative repeaters. The gateway 1 is bidirectionally coupled to a terrestrial communications system, such as the Public Switched Telephone Network (PSTN) which can provide either circuit switched or packet switched (e.g., Internet) voice and data access, as well as to a Ground Data Network (GDN) which provides an ability to communicate with a Ground Operations Control Center (GOCC) and other gateways, which are not shown. The gateway 1 includes typically a plurality of directional antennas, e.g., antennas 1A and 1B, while the user terminal 3 includes, typically, an omni-directional antenna 3A. For the case of a voice or data call that is connected to the PSTN the gateway 1 can provide multiple satellite diversity by routing Code Division Multiple Access (CDMA), or other appropriately modulated signals, forward links through both SAT 1 and SAT 2. Each forward link (FL) has a unique spreading code that enables the link to be separately despread, demodulated and combined in the user terminal 3, or other means of separating the received signals, so that they can be combined, may be used. This provides multiple satellite diversity for the user terminal 3, and facilitates overcoming signal blockage and fading conditions that may occur between the user terminal 3 and one or more of the satellites. The user terminal 3 may thus include a multi-finger decorrelator, such as a Rake receiver, or some other suitable receiver capable of discriminating the two or more signals. Two or more fingers of the Rake receiver can be used for receiving forward traffic channels from two or more of the satellites, while another finger can be time multiplexed to receive pilot signals transmitted by the gateway 1 through the different satellites. If available, a plurality of fingers can be used for this purpose. In one embodiment the user terminal derives link quality information from the forward links, such as signal strength, and transmits link quality indications (QIs) back to the gateway 1. In response to the received link quality indications the gateway 1 is enabled to selectively turn links on and off and/or control individual link power for signals transmitted through individual ones of the satellites 1A and 1B.
It would be desirable to extend these teachings so that a user terminal would be enabled to transition from a first gateway coverage area to a second gateway coverage area, during a call, without dropping the call or requiring that the call be somehow switched from the first gateway to the second gateway. For example, it is envisioned that during a call some mobile user terminals, such as those contained within automobiles, trains and aircraft, will cross political and other boundaries that also happen to define boundaries between adjacent gateway service areas.
It is a first object and advantage of this invention to provide a satellite communication system that enables a user terminal to simultaneously use satellites that are in view of at least two gateways.
It is another object and advantage of this invention to extend the teachings of the above-referenced U.S. patent applications Ser. No. 08/903,166, filed Jul. 3, 1997, and Ser. No. 08/239,750, filed May 9, 1994, to provide multi-gateway diversity for a user terminal, as well as to provide link control in the multiple satellite diversity case.
It is one further object and advantage of this invention to provide a technique for increasing an effect gateway coverage area.
It is another object and advantage of this invention to provide a satellite communications system that enables a user terminal to transition, during a call that is connected to a first gateway, from the service area of the first gateway to the service area of a second gateway without dropping the call and without requiring that the call be switched from the first gateway to the second gateway. In another implementation, not only the transition from the first gateway to the second gateway is implemented but, in addition, the call input to a first gateway switch is reconfigured so as to begin at the second gateway.
The foregoing and other problems are overcome and the objects are realized by methods and apparatus in accordance with embodiments of this invention.
In one aspect this invention provides a method for operating a satellite communications system, and a system that operates in accordance with the method.
The method includes steps of (a) establishing a call connection between a terrestrial telecommunications network and a user terminal via a first gateway and at least one satellite; and (b) while the call connection is established, coupling the user terminal to the terrestrial telecommunications network via a second gateway, in which during a period of time (which could be for the length of the call) the user is connected to both gateways, and at least one further satellite. During the coupling step, call speech or data information is conveyed between the first gateway and the second gateway over an inter-gateway communications link, in which during a period of time (which could be for the length of the call) the user is connected to both gateways, and data and/or voice traffic is conducted to the user simultaneous via two gateways.
The step of coupling includes a first step of detecting at the first gateway that the user terminal is capable of communicating with the at least one further satellite that is in view of the second gateway and the user terminal, a second step of negotiating with the second gateway for an assignment of resources to establish a connection with the user terminal, and a third step of bidirectionally coupling the terrestrial telecommunications network to the user terminal through the first gateway, through the second gateway, and through the at least one further satellite using the assigned resources. The at least one further satellite could be the at least one original satellite that is fading from the first gateway but not from the second gateway. The assigned resources in one instance uses the same resources (frequencies, power and other) that are assigned by the first gateway. In another implementation the user links are established at the second gateway with new and possibly unique resources.
The step of bidirectionally coupling can include a step of maintaining the call connection through the at least one satellite, or a step of terminating the call connection through the at least one satellite.
The step of detecting includes steps of generating received signal quality indications in the user terminal for established traffic channels and for pilot channels, and transmitting the quality indications back to the first gateway. A further step selectively controls forward link power and on/off states in accordance with the quality indications received from the user terminal.
Also disclosed is a method for providing a gateway with a larger effective terrestrial coverage area. This method includes steps of (a) providing a first gateway and a second gateway, each gateway having an associated terrestrial coverage area and each gateway being coupled to the same or a different terrestrial telecommunications network. In the case of the same network, the first and second gateways are coupled to the network at two different nodes. A next step (b) establishes a call connection between the terrestrial telecommunications network coupled to the first gateway and a user terminal located in the terrestrial coverage area of the first gateway by coupling the user terminal to the first telecommunications network via at least one satellite that is in view of the first gateway and the user terminal; and (c) while the call connection is established, further coupling the user terminal to the terrestrial telecommunications network via the second gateway and at least one further satellite that is in view of the second gateway and the user terminal. In one implementation the call is connected from the network (e.g., PSTN) to the first gateway and remains so for the duration of the call, in which case the call traffic data is conducted from the first gateway to the second gateway by a further network (which could be the ground data network). In another implementation, the call traffic is transferred to the second gateway during the call, while still maintaining the user connection to both the first and second gateways.
This method includes a further step of maintaining the call connection between the first terrestrial telecommunications network coupled to the first gateway and the user terminal after the user terminal moves from the terrestrial coverage area of the first gateway into the terrestrial coverage area of the second gateway.
The steps of coupling, further coupling, and maintaining each include a step of exchanging call-related data between the first gateway and the second gateway over the intergateway communications link.
In a further aspect of this invention the user can be provided with the xe2x80x9cbestxe2x80x9d quality of service of all satellites in view of both gateways.
In a preferred embodiment the forward and return links between the user terminal and a given gateway are independent, and the gateway diversity could be used on one link and not the other.